How Do You Fix 'not Defined' In Python

NameErrors are one of the well-nigh mutual types of Python errors. When you're first getting started, these errors can seem intimidating. They're not too complicated. A NameError means that you've tried to utilize a variable that does not yet exist.

In this guide, nosotros're going to talk virtually the "nameerror proper name is not defined" fault and why information technology is raised. We'll walk through a few example solutions to this error to help you lot sympathise how to resolve it in your code.

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What is a NameError?

A NameError is raised when yous try to use a variable or a role name that is not valid.

In Python, lawmaking runs from top to bottom. This ways that yous cannot declare a variable later you endeavour to use it in your code. Python would non know what you wanted the variable to do.

The about common NameError looks like this:

nameerror proper name is non defined

Let'south analyze a few causes of this error.

Cause #1: Misspelled Variable or Role Proper noun

It's easy for humans to gloss over spelling mistakes. We can easily tell what a word is supposed to exist even if information technology is misspelled. Python does not accept this capability.

Python can only interpret names that you have spelled correctly. This is because when you declare a variable or a function, Python stores the value with the verbal name you take alleged.

If there is a typo anywhere that y'all effort to reference that variable, an mistake volition exist returned.

Consider the post-obit lawmaking snippet:

books = ["Near Dark", "The Order", "Where the Crawdads Sing"]  print(boooks)

Our code returns:

Traceback (most contempo call last):   File "main.py", line 3, in <module> 	print(boooks) NameError: name 'boooks' is not divers

To solve this problem, all nosotros have to practise is fix the typo. If we apply "impress(books)", our code returns:

["Near Dark", "The Order", "Where the Crawdads Sing"]

If you receive a name error, you should first check to make sure that yous accept spelled the variable or function name correctly.

Cause #two: Calling a Function Before Declaration

Functions must be declared earlier they are used, similar variables. This is because Python reads code from top-to-bottom.

Let's write a programme that calls a function before it is declared:

books = ["Near Night", "The Order", "Where the Crawdads Sing"]  print_books(books)  def print_books(books): 	for b in books: 		print(b)

Our lawmaking returns:

Traceback (most recent call last):   File "master.py", line 3, in <module> 	print_books(books) NameError: name 'print_books' is not defined

Nosotros are trying to call print_books() on line three. Nonetheless, we do non define this function until later in our program. To ready this error, we tin movement our function declaration to a place before we apply it:

def print_books(books): 	for b in books: 		print(b)  books = ["About Dark", "The Order", "Where the Crawdads Sing"]  print_books(books)

Our code returns:

Near Dark The Lodge Where the Crawdads Sing

Our code has successfully printed out the list of books.

Cause #3: Forget to Define a Variable

Every bit programs get larger, it is like shooting fish in a barrel to forget to ascertain a variable. If y'all do, a name mistake is raised. This is because Python cannot piece of work with variables until they are declared.

Permit's accept a look at a plan that prints out a list of books:

Our code returns:

Traceback (nearly contempo telephone call last):   File "main.py", line one, in <module> 	for b in books: NameError: proper name 'books' is not defined

We take not alleged a variable called "books". To solve this problem, we need to declare "books" before we utilise information technology in our code:

books = ["Virtually Dark", "The Order", "Where the Crawdads Sing"]  for b in books: impress(b)

Let's try to run our plan once more and see what happens:

Near Dark The Order Where the Crawdads Sing

Now that we accept divers a list of books, Python can print out each book from the list.

Cause #four: Endeavor to Print a Unmarried Word

To print out a discussion in Python, yous demand to surround it in either unmarried or double quotes. This tells Python that a word is a string. If a word is not surrounded by quotes, it is treated as part of a program. Consider the post-obit print() argument:

This code tries to print the word "Books" to the panel. The code returns an mistake:

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Traceback (most recent call final):   File "main.py", line 1, in <module> 	print(Books) NameError: name 'Books' is non divers

Python treats "Books" like a variable name. To solve this error, we can enclose the discussion "Books" in quotation marks:

Python now knows that we want to print out a string to the console. Our new code returns: Books.

Cause #5: Declaring a Variable Out of Telescopic

In that location are two variable scopes: local and global.

Local variables are but accessible in the function or grade in which they are declared. Global variables are accessible throughout a program.

If you lot effort to access a local variable outside the scope in which it is defined, an error is raised.

The following code should print out a listing of books followed by the number of books in the listing:

def print_books():   books = ["Near Dark", "The Lodge", "Where the Crawdads Sing"]   for b in books:       print(b)  print_books() impress(len(books))

Our lawmaking returns:

Near Dark The Social club Where the Crawdads Sing Traceback (most contempo phone call last):   File "primary.py", line half-dozen, in <module> 	impress(len(books)) NameError: proper name 'books' is non defined

Our code successfully prints out the list of books. On the terminal line of our code, an mistake is returned. While we have declared the variable "books", we only declared it inside our print_books() office. This means the variable is non accessible to the residual of our program.

To solve this problem, we tin declare books in our main program. This will make it a global variable:

books = ["Near Dark", "The Lodge", "Where the Crawdads Sing"]  def print_books():   for b in books:       print(b)  print_books() impress(len(books))

Our code returns:

Near Nighttime The Guild Where the Crawdads Sing 3

Our lawmaking prints out every volume in the "books" listing. Then, our code prints out the number of books in the list using the len() method.